Drive units of the generic type in question are used to operate accessory devices in motor vehicles. When used for instance in motor vehicle windshield wiper systems, the drive shaft, formed by the armature shaft and the worm shaft, are subjected to an alternating axial load during operation, which varies from 0--when the wiper blades are at the turning points--to a maximum--when the wiper blades are being accelerated on leaving the turning points or being slowed down on approaching them. This axial load induced into the drive shaft by the worm gear leads to undesired knocking noises in the drive unit, if the drive shaft has longitudinal play between an axial stop toward the motor and an axial stop toward the gear.
For long-term elimination of this drive shaft longitudinal play, in a commercially available drive unit a threaded bore has been disposed on the side toward the worm shaft, penetrating the unit housing in the extension of the drive shaft axis of rotation, and a threaded pin is rotated in this bore. The threaded pin has a central blind bore, open toward the drive shaft, in such a helical compression spring is disposed that presses with prestressing against a mushroom-shaped stop placed on the face end of the shaft toward it. The prestressing of the helical compression spring, which belongs to the spring means, must be greater than the maximum axial load acting on the drive shaft, which load, because of the existing worm pitch direction, is always oriented toward the free end of the worm shaft portion of the drive shaft. This known type of eliminating drive shaft longitudinal play is cost-intensive, because of the disposition of the threaded bore in the unit housing. The known version also requires at least one manual and hence expensive assembly step.